1. Field of the Invention
The invention is related to the instrumentation technology field and more particularly to airspeed indicators for aircraft or wind tunnel applications.
2. Description of Prior Art
There are several types of airspeed and fluid speed indicators well known in the art. The most common devices are pitot tube systems using total and static pressure measurements. Pitot devices are extensively used in both aircraft and wind tunnels and also in boats and water tunnels.
Pitot devices are fairly rugged and provide satisfactory results for steady flow conditions where there are no rapid changes in flow speed and where the flow direction is fixed. Inaccurate and unsatisfactory results occur, however, where the flow changes direction so as to enter the total pressure port at an angle. Additionally, the response time of the pitot system prevents measurement of rapidly changing airspeeds, such as those encountered during oscillatory or unstable flow.
Another type of flow speed measurement system avoids the directional problem of a fixed pitot tube allowing steady state flow measurement in a particular plane such as the horizontal plane. These devices are commonly known as vane and cup anemometers and use the kinetic energy of the fluid to rotate mechanical components such as vanes or cups. Although these devices can measure flow speed accurately from any direction with the plane of rotation, the inertial characteristics of the rotating components prevent accurate measurement of rapid changes in speed. Rotary anemometers are also unsuitable for high speed flows and occupy a relatively large space.
Combination conventional instrumentation such as those using total-static pressure probes for airspeed, conical or hemispherical probes for flow angles, and "flying cruciforms" for establishing local flow angles and Mach numbers has poor frequency response characteristics and is inherently unsuitable for three-dimensional and unsteady flows. This type of flow is typical of conditions encountered in close proximity to full scale aircraft inlets in both confined and free jet flows.
A further type of instrumentation for wind tunnel use is based on optical devices such as laser velocimeters. These devices can provide accurate response to rapid changes in both direction and magnitude of a flow field, but require complex and expensive optical alignment systems and are generally unsuited for in-flight systems.
Another type of sensor system uses multi-element hot-wire or hot film anemometry. These sensors can provide good frequency response but they are usually fragile, sensitive to temperature changes and require accurate and repeated in-situ calibration for satisfactory performance.
The limitations of state-of-the-art instrumentation prevents the real-time measurement of unsteady airflow velocity except in limited test conditions such as wind tunnel model testing. Accurate real-time measurements of flow speed and angle needed for in-flight control of jet engine inlets, compressor guide vanes and rotors and other surfaces requiring active control are not available. Similarly accurate measurements needed in reduced stability aircraft are likewise available. As a result, unnecessarily large safety margins must be built in to prevent inlet unstarts, compressor stalls or loss of control of the aircraft. These larger safety margins reduce the available performance of the specific component and ultimately degrade aircraft performance.